I recently bought a used Rockshox Pike RC with 150mm of travel and the seller stated that the lower legs were not structurally sound and that the whole left leg was scratched up but that the rest of the upper components were fine. I was wondering if it is possible to buy new lower legs and replace the old ones keeping the upper components?

Huh?
So why did you buy it?
After paying for it, put the whole thing in the trash and buy a new fork.

Rockshox makes separate lowers for each of their forks.you best bet is to contact RS and find out exactly which lowers are right for your bike. then you need to find out where they are available, or if they are available at all. if you're not familiar with rebuilding forks, it might be best to send the fork off to a suspension specialist, but unless the fork was really cheap, this is going to boost the overall cost of the fork over what a new, undamaged one would have cost.

Bet you can but you will need new bushings. Not sure of the year and wheel side. May take some creative Google foo. Look for the manual. There may be a list of parts with part numbers. Here is one for a 26er on sale. There also may be some rebuild shops that would know where to get the form lowers.

https://www.bikeparts.com/BPC161390/...gaAg8rEALw_wcB

Yes you can. They are not cheap.

https://www.worldwidecyclery.com/pag...?q=pike+lowers

This is one of RS's nicer models so all the pieces can be ordered. That's fortunate, the cheaper forks are not like that and you can only get rebuilds.

The new lowers come with bushings in them.

Whoa those are expensive! I replaced the lowers on an older pike, and that were like $140.